Bronchodilators

Air reaches the lungs through branching tubes — the bronchi and bronchioles — wrapped in a layer of smooth muscle. When that muscle contracts, the tube narrows (bronchoconstriction) and airflow drops, felt as wheezing, chest tightness, and breathlessness. In asthma, the narrowing is largely reversible — bronchospasm layered on chronic airway inflammation. In COPD, airflow limitation is more fixed, and the reversible part is driven mostly by cholinergic (vagal) tone.

The width of the airway is set by a tug-of-war between the two halves of the autonomic nervous system. The sympathetic side, acting on β2 receptors on airway smooth muscle, tells the muscle to relax → the airway widens. The parasympathetic side, releasing acetylcholine onto M3 muscarinic receptors, tells it to contract → the airway narrows. Bronchodilators work on both ends of that rope: β2-agonists pull toward relaxation; anticholinergics block the constrictor signal.

A β2-agonist binds the β2 receptor and switches on a Gs → adenylyl cyclase → cyclic AMP (cAMP) → protein kinase A cascade. The rise in cAMP lowers intracellular calcium and relaxes the smooth muscle — bronchodilation — and also stabilizes mast cells so they release fewer inflammatory mediators. The clinically vital distinction is timing. Short-acting β2-agonists (SABAs) like albuterol work within ~5 minutes and last 4–6 hours — the rescue inhaler you reach for during an attack or before exercise. Long-acting β2-agonists (LABAs) like salmeterol last ~12 hours and are scheduled maintenance — they are not for a sudden attack.

Here is the rule students must lock in: a LABA must never be used alone in asthma. A LABA opens the airways but does nothing for the underlying inflammation — so it can relieve symptoms while the disease silently worsens, and used as monotherapy it was linked to an increase in asthma-related deaths. In asthma, a LABA is always paired with an inhaled corticosteroid, usually in a single combination inhaler. The one recognized exception to the never-a-reliever rule is a single ICS-formoterol inhaler used for both daily maintenance and quick relief (single maintenance and reliever therapy, SMART), because formoterol acts quickly; salmeterol, with its slower onset, is not suitable for this. (Anticholinergics such as ipratropium and long-acting tiotropium block the M3 constrictor signal and are workhorses in COPD.)

Although we call them "selective," β2 receptors aren’t confined to the lung, and the side effects are simply the drug hitting them elsewhere. In the heart, β-stimulation raises the rate → tachycardia and palpitations. On skeletal muscle, β2 activation causes a fine tremor (the most common complaint). And, exactly as insulin does, β2 stimulation of the Na⁺/K⁺-ATPase drives potassium into cells → hypokalemia — usually a redistribution, not a true deficit, but it can be significant with high-dose or nebulized therapy and is worsened by diuretics.

The same pathway also promotes glycogenolysis, releasing glucose and causing hyperglycemia (watch in diabetes). Two special dangers: paradoxical bronchospasm — the inhaler unexpectedly tightening the airway, often with the first use of a new canister, which means stop it immediately — and the quieter red flag of overusing a rescue inhaler. Needing albuterol more than about twice a week (or burning through canisters) doesn’t mean the SABA is failing; it means the underlying asthma is poorly controlled and the anti-inflammatory controller needs to be stepped up.